The Worst Worm

Most human parasites are well known and undeniably nasty. But a report last June in the British medical journal The Lancet turned up a new and especially fierce one. The story began like this: An aids patient in California visited his doctor complaining of weight loss and belly pain. ct scans revealed a large mass in his abdomen. A snip of tissue from the growth turned up cells that a host of experts could not identify. Nine weeks later, the patient died.

At autopsy, doctors discovered that three-quarters of the man’s liver was gone. Several growths were removed, including an eight-inch tumor from his abdominal cavity. About a tenth of the cells in the tumor looked very strange for cancer cells. They were too small to be human, says Luis Fajardo, a Stanford pathologist and one of the authors of the Lancet report. Moreover, they contained plenty of silicon, which is extremely rare in human cells.

Further study of the tissue turned up saclike structures filled with many cells. Still, they didn’t bear a clear resemblance to any known pathogen. Finally microbiologist David Relman of the Stanford medical school analyzed DNA from the sac cells, selecting one familiar gene and comparing it with the versions of that gene existing in other organisms. He did not find an exact match, but he did find a strong resemblance to a gene in the tapeworm family.

That genetic resemblance proved to be the only solid clue: the researchers now believe they may have discovered a new and uncommonly awful tapeworm. Maybe this is a parasite that only infects individuals that are immunodeficient, says Fajardo. But it was so aggressive that one worries whether this parasite can also infect other individuals.